Dual polarized antennas are used in a wide range of applications, such as radar and radiometer systems (ground based as well as aircraft and satellite borne), systems for reception of satellite TV, radio links, data transmission networks (LAN and WAN). Typically, the operating frequency of such antennas is within the range from 1 GHz to 100 Ghz (microwave and millimeter waves).
Single polarized antennas, i.e. antennas radiating electromagnetic waves of a single polarization, are also used in a broad range of applications, such as in cellular radio and other personal communication systems operating in the VHF, UHF and microwave frequency range (e.g. L and S band).
Dual polarization antennas of the planar type are more and more commonly used for reception of satellite TV, typically, because of the possibility of frequency reuse, i.e. two TV channels may be transmitted simultaneously on the same frequency from the same satellite or from closely spaced satellites, with orthogonal polarization. Due to the orthogonality, the two channels can be received independently provided that the receiving antenna has the required low cross polarization between the two polarizations so that the two signals can be discriminated without mutual interference. Due to the increasing amount of wireless data communication throughout the frequency spectrum, it is expected that antennas with low cross polarization will gain wider use in the near future, first of all because of the possibility of doubling the data transmission capacity within a specific frequency range by utilization of orthogonal polarizations of the transmitted electromagnetic waves, and secondly because of the fact that some wireless data communication systems, such as high speed data communication systems utilizing dual polarizations, are sensitive to mutual interference, the sensitivity can be reduced by adopting antennas with low cross polarization.
Also, transmission of signals to or from mobile/portable radios may be enhanced by transmission of dual polarized signals to mobile/portable antennas with low cross polarization as the possibility of signal drop outs may be decreased. Signal drop outs are caused by the fact that signals received at the mobile/portable antenna, typically, have propagated to the antenna along multiple paths, e.g. due to reflections, e.g. by buildings. Signals of a given polarization travelling along different paths may then cancel each other at specific positions of the mobile/portable antenna depending upon the phase and amplitude relationship of the signals at different positions. However, as phases typically differ for signals of different polarizations, a signal drop out caused by cancellation of the signal of one polarization may be eliminated by switching of the receiver to the signal of the other polarization.
Dual polarized microstrip antenna arrays comprising one or more resonant radiating or receiving patches are known in the art. Typically, the resonant radiating or receiving patches are square shaped, the side of the square being substantially equal to one half wavelength at the transmitting and/or receiving frequency as measured in the dielectric of the microstrip antenna element. Each patch of the array is connected to a feeding network for transmission of a signal to be radiated by the patch, or, for transmission of a signal received by the patch to a receiver. Each patch is, for example, fed from one side of the patch for excitation of electromagnetic radiation of a polarization orthogonal to the side of the patch. A feed line connected to an adjacent orthogonal side of the square can then be utilized to excite electromagnetic radiation of an orthogonal polarization.
Although there is a degree of isolation between such prior art dual polarized microstrip antennas, there is also unavoidable coupling between the input/output ports. Typically, such feed through is on the order of -25 dB which is undesirably high for many applications.
In U.S. Pat. No. 4,464,663, it is disclosed how to enhance isolation between input/output ports for two differently polarized signals to be radiated by or to be received from a microstrip antenna of the type having integral microstrip feed lines and resonant radiating patches of the above mentioned kind by utilization of dual polarized radiating patches in pairs with one of the polarized feeds being provided back-to-back between the spaced apart pair of patches by a feed line system that incorporates a 180.degree. phase difference of the feeding signals.
In Granholm J., Woelders, K., Dich, M., and Christensen, E. L., "Microstrip Antenna for Polarimetric C-band SAR", IEEE AP-S International Symposium and URSI Radio Science Meeting, Seattle, Wash., Jun. 19-24, 1994, pp. 1844-1847, a 224 element dual linearly polarized microstrip array antenna with low cross polarization that also utilizes dual polarized radiating patches in pairs is disclosed.
It is a disadvantage of known techniques for suppression of cross polarization in dual polarized antenna arrays that the side lobe suppression is insufficient for many applications. It is a major disadvantage of prior art techniques for suppression of cross polarization that undesired grating lobes in the radiation pattern of an antenna array are generated for antenna arrays with many antenna elements, e.g. with more than 6-8 antenna elements.
As further explained below, grating lobes are undesired side lobes in the radiation pattern of an antenna array.